Optical bond-wire interconnections and a method for fabrication thereof

ABSTRACT

Optical bond-wire interconnections between microelectronic chips, wherein optical wires are bonded onto microelectronic chips. Such optical connections offer numerous advantages compared to traditional electrical connections. Among other things, these interconnections are insensitive to electromagnetic interference and need not be located at the edges of a chip but rather can be placed for optimal utility to the circuit function. In addition, such interconnections can be given the same or other pre-specified lengths regardless of the placement in the module and they are capable of signal bandwidths up to 20 Gigahertz without causing a cross-talk problem. A method of fabrication of such optical interconnections using optical fiber, a laser or photodetector and etched mirror and etched V-shaped grooves.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application is a divisional application of U.S. Ser. No.10/649,075, filed on Aug. 26, 2003, which is a divisional of U.S. Pat.No. 6,655,853 issued on Dec. 2, 2003.

I. BACKGROUND OF THE INVENTION

1. Field of the Invention.

This invention relates to the interconnection of microelectronic chips.The interconnections may be between chips on a multi-chip module,between several multi-chip modules, or even between distant points on alarger chip. More particularly, it pertains to the use of optical wiresbonded on those chips to interconnect the chips. The interconnections bymeans of optical fibers are made to substitute for the electrical wireinterconnections. Each optical wire terminates at a small laser chipleton one end and a photodetector chiplet on the other end. Each chiplet isflip-chip mounted onto the larger electronic chips and each contains avertically coupled laser or photodetector and solder bumps on one faceand a deflecting mirror and a V-groove (into which one end of theoptical fiber is inserted) on the opposite face.

2. Description of the Related Art.

In a high speed multi-chip module (MCM) environment, chip-to-chipconnections are usually made using bond wires, with microstrip lines onthe MCM substrate used to interconnect chips that are farther apart.

Presently, electrical bond wires are used to interconnect microchips.Using the electrical wires has serious drawbacks. The electrical wiresare sensitive to electromagnetic interference and themselves create suchinterference which poses especially serious problems for distribution oftiming signals. The electrical wires must be located at the edges ofchips. Signal attenuation and phase delay depend upon the length of theelectrical wires. Thus, depending on the lengths of the electrical wiresand their locations in the module, it may be difficult to achieve equalattenuation and/or equal signal phase delay among multiple wires, ifneeded.

In addition, in many cases signal bandwidths of several Gigahertz aredesirable but cannot be achieved if electrical wires are used becauseelectrical bond wires act as open antennae at high frequencies andintroduce noise coupling among the wires. For example, bond wires of 500micrometers in length and 1 mil (0.001 inch) diameter carrying 10milliamperes of current will produce appreciable (100 millivolts ormore) coupling or cross-talk at 10 Gigahertz even when they are spacedseveral pitch distances apart, a typical pitch being 100 to 150micrometers. This effect will substantially limit the maximum speed of atypical MCM module having hundreds of bond wires from several chips. Thecross-talk is even more severe when the chips are located farther apartand require longer bond wires.

Therefore, there is a need to have interconnects between microchipswhich:

-   -   (a) are insensitive to electromagnetic interference;    -   (b) need not be located at the edges of a chip but rather can be        placed for optimal utility to the circuit function;    -   (c) can be given the same or other pre-specified lengths        regardless of the placement in the module; and    -   (d) are capable of signal bandwidths up to 20 Gigahertz without        causing the cross-talk problem.

Optical bond-wire interconnections satisfy all these requirements.Previously, optoelectronic devices such as vertical-cavity lasers andphotodetectors have been bonded onto microelectronic chips to providefree-space optical interconnections and the results were reported, forinstance, by D. A. Louderback, et al., in “Modulation and Free-SpaceLink Characteristics of Monolithically Integrated Vertical-Cavity Lasersand Photodetectors with Microlenses”, IEEE Journal of Selected Topics inQuantum Electronics, Vol. 5, No. 2 (1999).

However, for such free-space interconnections, the optoelectronicdevices must be installed in a way that they face one another. Moreover,their relative locations must be precisely controlled to ensure opticalalignment. As a result, in free-space optical interconnections, theoptoelectronic devices must be located on different multi-chip modulesthat are held in immediately adjacent slots of a rack.

With optical interconnect wires bonded directly onto microelectronicchips there is almost no constraint on the locations of the chips to beinterconnected. The chips may reside on the same multi-chip module ormay be disposed many meters apart. These chips can even be members ofdifferent instruments or computation units; however, if theoptical-fiber bond-wire is subject to movement, then some mechanicalmeans is preferably provided to relieve the optical fiber and chipletsfrom excessive strain.

In the prior art, optical fibers are typically coupled to optoelectronicdevices using an accompanying sub-mount, such as a machined piece ofmetal, or ceramic, or a V-grooved silicon substrate, when both theoptical fiber and the optoelectronic device chip are mounted on thesub-mount. Directly attaching and optically aligning an optical fiber toan optoelectronic chip would be most beneficial.

There exists no known prior art for fiber-based optical interconnectsbonded directly onto microelectronic chips. Yet, as discussed above, theneed for such is acute.

For the foregoing reasons, there is a necessity for optical bond-wireinterconnections. The present invention discloses such interconnections.

II. SUMMARY OF THE INVENTION

The present invention is directed to an optical bond-wire interconnectand to a method of manufacturing of the interconnect. It can be usedinstead of electrical bond-wires but can be much longer than theelectrical bond-wires. For instance, length of an electrical wiretypically does not exceed maximum length of 1 centimeter and is usuallyshorter. An optical bond-wire can reach lengths of the order of hundredsof meters.

Each optical bond-wire comprises a segment of optical fiber that isattached at its two ends by means of terminations to the microelectronicchip or chips. The two terminations of the optical bond-wire are a laserchiplet on one end of the optical bond-wire and a photodetector chipleton the other end. Each chiplet can be as small as 250 by 250 micrometersand is connected to two electrical lines—one line is the signal to besent via the interconnect and the other line is a return or ground.

III. BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present invention will be betterunderstood with regard to the following description, appended claims,and accompanying drawings where

FIG. 1 is a schematic diagram showing an optical bond-wireinterconnection for electrical signals.

FIG. 2 is a schematic diagram showing a method for connecting an opticalfiber to the optoelectronic termination laser or photodetector.

FIG. 3(a) is a picture of an etched mirror fabricated by wet-chemicaletching into gallium-arsenide.

FIG.3(b) is a picture of a V-shaped holder fabricated by wet-chemicaletching into gallium-arsenide.

FIGS.4(a)-4(c) are schematic diagrams showing an example of the opticaldesign for an optoelectronic termination.

FIGS. 5(a)-5(g) are schematic diagrams showing step-by-step method offabrication of an optical bond-wire interconnection of the presentinvention.

IV. DETAILED DESCRIPTION OF THE INVENTION

A preferable optical bond-wire interconnection for electrical signals isschematically illustrated on FIG. 1. Optical bond-wires 3 may be used tointerconnect monolithic microwave integrated circuits (MMIC) 2, forexample. The MMICs can be located on the same multi-chip module (MCM) 1or on different MCMs. As can be seen from FIG. 1, the length of opticalbond-wires 3 can be substantially longer than that of electric wires 4.There is no need for MMICs to be immediately adjacent in the case ofoptical bond-wire interconnections. As pointed out above, the length ofoptical bond-wires can reach hundreds of meters.

A segment of optical fiber is attached at its two ends by means ofterminations to the microelectronic chips. The two terminations of theoptical bond-wire are a laser chiplet on one end of the opticalbond-wire and a photodetector chiplet on the other end. A connection ofthe optical bond-wire 3 to an optoelectronic termination is shown onFIG. 2. The termination comprises a laser or photodetector 5 which isoptically coupled to the optical fiber 3 and electrically coupled to theMMIC 2. Both the laser and photodetector are edged coupled devices or,preferably, vertically coupled devices, such as vertical-cavity surfaceemitting laser (VCSEL) and PIN photodiode detector (hereinafter, PIN) ormetal-semiconductor-metal (MSM) photodetector, respectively.

The optical fiber 3 is preferably mounted onto the backside of theoptoelectronic laser or photodetector chiplet 8. Other, non-preferredmethods of mounting the optical fiber 3 include mounting where theoptical fiber comes directly from the top of chiplet 8 and mounting byetching an opening in the back of the substrate followed by directinsertion of the optical fiber.

After the optical fiber 3 has been mounted onto the backside of theoptoelectronic laser or photodetector chiplet 8, it is attached and heldin place, preferably, using adhesives such as UV-curable epoxy resinscommonly known to those reasonably skilled in the art.

The optical path is defined in such a way that light is deflected fromthe optical fiber bond-wire 3, through the substrate and to thephotodetector 5, or vice versa in the case of the termination being thelaser 5. The substrate materials of the chiplets 8 are preferablygallium arsenide or indium phosphide.

Electrical signals and power to and from the optoelectronic chip 8 aredelivered via, preferably, solder bumps 9. As an alternative to thesolder bumps 9, gold/gold compression bonds can also be used instead. Achiplet 8 comprises at least three, and preferably four, solder bumps 9,at least two of which are electrically connected to the microelectronicchip or the MMIC 2.

A preferred kind of the solder bumps is precision electroplated solderbumps disclosed in U.S. patent application Ser. No. 09/522,803,currently pending. Other kinds of solder bumps commonly used by thosereasonably skilled in the art may also be used. The solder bumps 9 alsoprovide mechanical support for the chiplet 8 and its physical adhesionto the MMIC 2.

Standard methods known to those reasonably skilled in the art are usedto fabricate the optical fiber 3 and to cleave the fiber into thedesired length for the bond-wire. Commercially available optical fiberis used. In particular, for the design shown on FIGS. 4(a)-4(c), amulti-mode optical fiber manufactured by Fiberguide Industries Corp. ofNew Jersey, is used. The optical fiber has a fairly large numericalaperture, preferably 0.35 or more.

Standard methods known to those reasonably skilled in the art are alsoused to fabricate the solder bumps 9, as well as the laser andphotodetectors 5. A method for fabrication of the preferred solderbumps, the precision electroplated solder bumps, is disclosed in U.S.patent application Ser. No. 09/522,803, currently pending.

Laser and photodetector units 5 are available from University ofCalifornia at Santa Barbara of Santa Barbara, Calif. The lasers andphotodetectors 5 to be used are those which operate at an opticalwavelength for which the substrate material of the chiplet 8 istransparent.

Suitable VCSELs are emitting at such wavelengths so that the selectedsubstrates be transparent and the signal be detectable by thephotodetectors. In particular, in case of gallium arsenide substrates,the preferred VCSELs are those emitting preferably at a wavelength ofabout 980 nanometers or about 1,300 nanometers and in case of indiumphosphide substrates—at a wavelength of about 1,300 nanometers or about1,550 nanometers.

For photodetectors, those that are sensitive in the range of wavelengthsbetween about 980 nanometers and about 1,550 nanometers are suitable forboth gallium arsenide and indium phosphide substrates.

The optical bond-wire interconnect further comprises, preferably, amirror 7 and a V-shaped groove 11, as shown on FIG. 2, for holding theoptical fiber 3. The face of the mirror 7 slopes downward and outwardfrom the surface of the wafer. At the same time, the walls of theV-shaped groove 11 slope downward and inward.

The mirror 7 and the groove 11 are preferably fabricated simultaneouslyand are positioned perpendicularly to each other. This perpendicularpositioning of the mirror 7 and the groove 11 is not required but isstrongly preferred. Depending on the design of the device, thosereasonably skilled in the art will modify the relative positioning ofthe mirror 7 and the groove 11 and may choose an angle other than 90°between them.

The process of such simultaneous fabrication is only possible due to thefact that both gallium arsenide and indium phosphide substrates, onwhich the mirror 7 and the groove 11 are formed, preferably have azinc-blende crystallographic structure. A consecutive fabrication of themirror 7 and the groove 11 is also possible, but the simultaneousfabrication is easier to achieve and allows automatic alignment, whichthe consecutive fabrication does not provide. The zinc-blende structureis not required but is preferred as it makes the fabrication processeasy. Those skilled in the art may modify the process and choose astructure other than the zinc-blende structure.

First, the laser or photodetector 5 is fabricated on a first side of awafer, or substrate 18, preferably, on a gallium arsenide or indiumphosphide substrate, as shown on FIG. 5(a). The process for suchfabrication comprises the epitaxial growth of the laser or photodetector5 material. An etch-stop layer 17 is grown first, underneath the devicelayers and on the substrate wafer 18. The second side of the wafer 18 isthen thinned and polished which step determines the depth of the mirrorchannel to be discussed below.

Next, the mirror 7 and the V-groove 11, as shown on FIG. 2, arepreferably formed on the back side of the wafer. Such step of theformation of the mirror 7 and the V-groove 11, as shown on FIG. 2,preferably comprises the following sub-steps.

First, a thin film of preferably silicon nitride 19 is deposited on theback side of the wafer, with the thickness preferably greater than about300 nanometers, as shown on FIG. 5(b). Following the deposition of thesilicon nitride film 19, a thin layer of photoresist 20 is deposited andpatterned on top of the silicon nitride film 19. The photoresistmaterial is a typical and commonly known material used by those skilledin the art and is applied according to well known techniques also knowto those reasonably skilled in the art. The thin film 19 of siliconnitride can be deposited by the method of plasma-enhanced chemical vapordeposition, by sputtering or by high temperature chemical vapordeposition.

As shown on FIG. 5(c), T-shaped openings are next patterned into thesilicon nitride film 19 by photolithographic techniques known to thosereasonably skilled in the art and standard wet or dry etching processesof the silicon nitride also known to those reasonably skilled in theart. The “T” shape of the openings is preferred, but some other shapes,for example “I” shape, are also possible.

The “T” on the back side of the wafer 18 is precisely aligned with thelaser or photodetector 5 formed on the top side followed by theformation of the mirror 7 and the V-groove 11, as shown on FIG. 2, inthe top and the trunk of the “T” respectively. The preciseness of thealignment is preferably within a few micrometers deviation and thealignment is achieved and measured using standard techniques andinstruments known to those reasonably skilled in the art.

The mirror 7 and the V-groove 11 are preferably formed by wet-chemicaletching, as shown on FIG. 5(d). A preferred etchant for both galliumarsenide and indium phosphide substrates 18 is about 2% solution ofbromine in methanol. An acceptable alternative etchant for galliumarsenide substrate 18 is a mixture of hydrogen peroxide and an acid,such as hydrochloric acid. The ratios between the components in theetchant mixtures are common and known to those skilled in the art.

Well-defined etched profiles and the smooth surfaces obtained with theuse of H₂ 0 ₂-HCl mixture are shown on FIG. 3. The mirror 7 and theV-groove 11, shown on FIG. 2, are precisely aligned along specificcrystallographic directions. For instance, if the back side of thesubstrate is a (100) crystallographic surface, the V-groove 11 isaligned along the (01) direction and the mirror 7 is aligned along the(O) direction. This crystallographic alignment can be accomplishedmanually or with the use of standard instruments according to standardtechniques known to those skilled in the art.

Reproducible etching is done by controlling the undercutting achieved byproper choice and fabrication of the thin-film masking material. Suchchoice and fabrication are known to those skilled in the art. FIG. 3also illustrates the amount of undercutting. Etching of the mirror 7 andthe V-groove 11 is continued until the etch-stop layer 17 is exposed.

After the formation of mirror 7 and the V-groove 11, the photoresistlayer 20 and the silicon nitride layer 19 are etched away using commonand known etching techniques. This step is followed by the fabricationof solder bumps 9 on top (first) side of the wafer 18, as shown on FIG.5(e).

Multiple chiplets 8 can be fabricated on one wafer 18 followed by thedicing of the chiplets 8 from the wafer 18 into separate laser chiplets51 or photodetector chiplets 52. Finally, the optical fiber 3 isinserted into the V-groove 11 and attached to a chiplet 8 as shown onFIGS. 5(f) and 5(g) for the laser chiplets and for the photodetectorchiplets, respectively.

EXAMPLE 1

An example of an optical design is shown on FIGS. 4(a)-4(c). Thisexample is introduced solely for the purposes of illustration of apossible design and is not to be construed a limitation.

The design shows relative positions of the optical fiber 3 and theoptoelectronic laser or photodetector 5. This design can be used tospecify the photolithographic fabrication masks and the etchingparameters.

For a bromine-methanol etchant described above, and under the etchingconditions used for this example, the resulting mirror 7 is inclined atan angle of about 55° and deflects the optical beam into the wafersubstrate. The resulting V-groove 11 has sidewalls inclined at an angleof also about 55°. The optical fiber 3 has a core diameter of about 100micrometers and a cladding diameter (not shown) of about 200micrometers. To ensure that the core of the optical fiber 3 is entirelybeneath the surface of the wafer 12, the optical fiber 3 is set intoV-groove 11 in such a way so that the optical fiber's 3 optical axis islocated at a depth 13 of about 60 micrometers.

The width 14 and the depth 15 of the V-groove 11 are about 384micrometers and 234 micrometers, respectively, as shown on FIG. 4(a).

Since the etching rate of the V-groove 11 slows considerably once thepoint of the “V” has been formed, the depth 16 of the mirror channel[FIGS. 4(a) and 4(b)] is greater than the depth 15 of the V-groove 11and is about 247 micrometers. An etch-stop layer 17, is preferably grownepitaxially.

The etch-stop layer 17 has a thickness within a range of between about0.01 nanometers and about 0.5 nanometers, preferably, within a range ofbetween about 0.02 nanometers and about 0.2 nanometers, and is made ofpreferably aluminum arsenide for the gallium arsenide substrate 18 andof preferably indium aluminum arsenide for the indium phosphidesubstrate 18.

When the mirror 7 is being etched, the etching is allowed to continueuntil the etch-stop layer 17 is reached. This technique ensures that thebottom surface of the mirror channel is flat and smooth because theetching would otherwise produce a non-flat surface at the bottom of themirror channel. Following the etching process, the etch-stop layer 17 isoptionally removed by a known method.

The distance between the etch-stop layer 17 and the light-emitting orlight-absorbing area of the optoelectronic device 5 is designed to bebetween about 1 and about 5 micrometers, preferably, about 3micrometers. The locations of the optoelectronic devices 5 areillustrated on FIG. 4(b) (for a photodetector) and FIG. 4(c) (for alaser). The space where the light travels can be sealed, for examplewith an epoxy sealant.

For typical applications where the optical bond wires 3 have lengths ofup to about several meters, and the signal bandwidth or pulse rate isbelow about 20 Gigahertz, a multimode optical fiber can be used,preferably having a numerical aperture of about 0.4. The fiber corediameter can be as low as 50 micrometers and the cladding diameter ispreferably between about 125 and about 200 micrometers.

The photodetector 5 should have a size at least as large as the fibercore in order to capture all the light from the fiber. The diameter ofthe laser 5 is preferably between about 10 and about 30 micrometers.

Having described the invention in connection with several embodimentsthereof, modification will now suggest itself to those skilled in theart. As such, the invention is not to be limited to the describedembodiments except as required by the appended claims.

1-5. (canceled)
 6. The method of claim 42, wherein the first and secondterminations each comprise a groove and a mirror.
 7. The method claim42, wherein said optical fiber is disposed within a first groove of thefirst termination and a second groove of the second termination. 8.(canceled)
 9. The method of claim 42, wherein said bonding comprisesusing solder bumps or gold/gold compression bonds.
 10. The method ofclaim 42, wherein an optical path is established so that light travelsfrom said laser to said photodetector.
 11. The method of claim 42,wherein said terminations are having a size of not more than about 250micrometers in width and not more than about 250 micrometers in length.12. The method of claim 42, wherein said laser comprises avertical-cavity surface emitting laser.
 13. The method of claim 42,wherein said photodetector comprises a PIN photodiode detector or ametal-silicon-metal photodetector.
 14. The to method of claim 6, whereinsaid mirror is disposed on a second side of said terminations.
 15. Themethod of claim 6, wherein a second side of said substrate is a (100)crystallographic surface, said groove is aligned along (011)crystallographic direction and said mirror is aligned along (011)crystallographic direction. 16-18. (canceled)
 19. The method of claim42, wherein said first and second substrates comprise a Zinc-blendecrystallographic structure. 20-25. (canceled)
 26. The method of claim42, wherein said laser emits at a wavelength selected from a group ofwavelengths, said group comprising wavelengths of about 980 nanometersor about 1300 nanometers.
 27. The method of claim 42, wherein said laseremits at a wavelength selected from a group of wavelengths, said groupcomprising wavelengths of about 1300 nanometers or about 1550nanometers.
 28. The method of claim 42, wherein said photodetector issensitive within a range of wavelengths between about 980 nanometers andabout 1550 nanometers. 29-41. (canceled)
 42. A method of opticallyinterconnecting at least two metallization points, the methodcomprising: selecting an optical wire comprising: a first terminationadapted to be placed directly onto a first metallization point, whereinthe first termination comprises a first substrate, a photodetectorconnected with the first substrate, and at least one first terminaladapted to electrically connect the first metallization point with thephotodetector; a second termination adapted to be placed directly onto asecond metallization point, wherein the second termination comprises asecond substrate, a laser connected with the second substrate, and atleast one second terminal adapted to electrically connect the secondmetallization point with the laser; an optical fiber opticallycontacting the first and second terminations; bonding the firsttermination onto the first metallization point; and bonding the secondtermination onto the second metallization point.
 43. The method of claim42, wherein the first metallization point is located in a firstmicroelectronic chip.
 44. The method of claim 43, wherein the secondmetallization point is located in the first microelectronic chip. 45.The method of claim 43, wherein the second metallization point islocated in a second microelectronic chip.
 46. The method of claim 42,further comprising: receiving an electrical signal from the secondmetallization point; generating an optical signal using the laser basedon the electrical signal; transmitting the optical signal through theoptical fiber; converting the optical signal into another electricalsignal using the photodetector; and transmitting the another electricalsignal to the first metallization point.
 47. The method of claim 43,wherein the photodetector is disposed on a first side of the firstsubstrate.
 48. The method of claim 43, wherein the laser is disposed ona first side of the second substrate.